1. Field of the Invention
This invention relates to a process for producing (-)-3a, 6,6,9a-tetramethyl-dodecahydronaphtho[2,1-b]furan (hereinafter "L-ambrox") represented by Formula (1): ##STR1## which is an important perfumery substance having an excellent aroma of an amber. More particularly, this invention relates to a process for producing the L-ambrox by the use of a (-)-2,5,5,8a-tetramethyl-1-(carboxymethyl)-2-hydroxydecalin (hereinafter "(-)HC acid") represented by Formula (10a): ##STR2## as a starting material, at a low cost and in an industrial scale. This invention also relates to a process for optically resolving (.+-.)-2,5,5,8a-tetramethyl-1-(carboxymethyl)-2-hydroxydecalin, which is a mixture of equal quantities of (-)HC acid and (+)-2,5,5,8a-tetramethyl-1-(carboxymethyl)-2-hydroxydecalin (hereinafter "(+)HC acid"), into (-)HC acid and (+)HC acid to thereby obtain the optically active (-)HC acid or (+)HC acid represented by Formula (10b): which is useful as a starting material for synthesizing a compound having a trimethyl-transdecalin skeleton represented by Formula (A): ##STR3## having a physiological activity or a characteristic aroma, as exemplified by the L-ambrox or the like.
2. Description of the Related Art
Optically active substances having a trimethyl-transdecalin skeleton naturally occur and many of such substances are known to have useful physiological activities or characteristic aromas [Von Gerhard Buchbauer et al., Chemiker Zeitung, 112, 319-333 (1988)]. For example, the optically active (-)-3a,6,6,9a-tetramethyl-dodecahydronaphtho[2,1-b]furan (common name: L-ambrox) represented by Formula (1) is known as an important substance having the aroma of an amber.
As processes for producing the L-ambrox, the following processes have been hitherto proposed.
Process (a)
An optically active substance (-)-sclareol present in an essential oil of naturally occurring Salvia sclarea is used as a starting material. This is subjected to chromium oxidation to produce (+)-norambreinolide [Helv. Chi,. Acta., 14, 570, (1931)], and then the (+)-norambreinolide is reduced, followed by cyclization [Dragoco Report, 11/12, 276-283 (1979)].
Process (b)
A process in which an optically active substance (+)-manool is used as a starting material (Japanese Patent Application Laid-open No. 62-39539).
Process (c)
A process in which an optically active substance L-abietic acid is used as a starting material [M. Ohno et al., Tetrahedron Letters, 28, 2863 (1987)].
Process (d)
A process in which an optically active substance L-levopimaric acid is used as a starting material [Y. Nishi et al., J. Jpn. Oil. Chem. Soc., 38, 276 (1989)].
The process (a), however, has been involved in the problem that the starting material used, Salvia sclarea oil, is unsuitable as the starting material for producing the L-ambrox in an industrial scale since it can be obtained from vegetables cultivated only in limited areas having particular climate or natural features and is relatively expensive. In addition, this process has also had the problem that a heavy metal oxidizing agent such as potassium permanganate or chromic acid, which is not suited for its industrial use because of the problem of environmental pollution caused by waste liquor, is used as an oxidizing agent when the (-)-sclareol is decomposed by oxidation.
The process (b) is reported to be a little more advantageous than the process (a) in view of the supply of starting materials, but has had the problem that a heavy metal oxidizing agent is also used in the step of oxidative decomposition.
The processes (c) and (d) are considered to be more advantageous than the processes (a) and (b) in view of the supply of starting materials, but has had the problem that it requires a number of reaction steps and use of expensive reagents and involves many reaction procedures unsuitable for their industrial application. In this regard, when a compound having the optically active trimethyl-transdecalin skeleton including L-ambrox is produced, asteric structure to which the optical activity thereof is attributable must be selectively realized for the compound. For that purpose, a variety of production processes have been hitherto proposed. For example, it is known to use a method in which the final compound is produced using an optically active starting material obtainable from natural sources or a method in which the desired optically active compound is produced by utilizing enzymatic reaction or other reaction similar thereto by the use of an optically inactive starting material.
However, the conventional method of producing the final compound by the use of an optically active starting material has been involved in the problem that optically active starting materials can be obtained with difficulty or the materials themselves are expensive. The method of producing the desired optically active compound by utilizing enzymatic reaction or other reaction similar thereto by the use of an optically inactive starting material has also had the problem that in many instances it is unsuitable for its industrial application because of a very low substrate concentration or in some instances the final compound obtained can not be said to have a satisfactory optical purity. In addition, in both the methods, they require a long course of steps until the materials are converted to the final end compounds and there are often reaction steps that are difficult to carry out in an industrial scale.
Thus, the conventional processes have had the problem that the compounds having the optically active trimethyl-transdecalin skeleton, including the L-ambrox, can not be produced at a high optical purity in an industrial scale.